The preliminary data from our laboratory established a novel and previously underappreciated role of the complement (C) system in modulating SDF-1-CXCR4 axis-dependent homing/retention/mobilization of hematopoietic stem/progenitor cells (HSPC) in bone marrow (BM). We found that C proteins are synthesized in BM and C activation provides one of the earliest signals of BM injury e.g., during conditioning for transplant by irradiation/chemotherapy or G-CSF-induced mobilization. We reported that HSPC express functional C3aR and the iC3b-receptor, CR3, and while C3a and desArgCSa stimulation of HSPC enhances/sensitizes responsiveness of HSPC to SDF-1 gradients, deposition of iC3b on BM stroma tethers early CR3-positive hematopoietic progenitors causing retention in the BM. Four aims are proposed to better elucidate the role of C3 in hematopoiesis during stress situations: 1. The molecular aspects of the C3a-C3aR axis in homing/engraftment of HSPC in bone marrow. Preliminary research points to a crucial role for the C3a-C3aR axis in homing/retention of HSPC in the bone marrow. We will focus on determining the molecular mechanisms of C3aR signaling in HSPC crucial for these processes and test whether priming of HSPC before transplantation with C3 cleavage fragments enhances engraftment of long-term repopulating HSPC in wt mice and human cord blood HSPC engraftment in NOD/SCID mice. 2. Blockade of the C3a-C3aR axis as a new strategy to mobilize stem cells. We reported that the small molecular C3aR antagonist SB2900157 enhances and accelerates G-CSF-induced mobilization. As a continuation of this work we will study the phenotype of mobilized cells and test whether they engraft efficiently after transplantation into syngeneic mice. 3. C3 fragments as modulators of the SDF-1-CXCR4 axis. C3a, desArgCSa and iC3b prime chemotaxis of HSPC to SDF-1. This aim will define the molecular mechanism of this phenomenon and explore the hypothesis that C3 cleavage fragments promote a shift of CXCR4 into membrane lipid rafts which increases their responsiveness to SDF-1. 4. C3 and impaired mobilization in immunodeficient mice. We have observed that immunodeficient RAG2-/- and SCID mice show impaired mobilization of HSPC, which is restored after infusion of immunoglobulins. We will test the hypothesis that this defect is related to the lack of naturally occurring C cascade-activating IgM antibodies.